Significance of macroporosity and hydrology for soil management and sustainability of agricultural production in a humid-tropical environment

This paper analyzes soil-related agronomic constraints in the Sitiung regio n of Indonesia that are directly related to low nutrient-holding capacity, macroporosity, and rainfall regime. This region receives 2500 to 3000 mm of rainfall per year; but nearly 50% of the rainfall is disposed of rapidly v ia internal drainage. Although rapid internal drainage reduces the risks of erosion, it leads to infertility, acidity, and Al toxicity. The physical s tructure of the soils is: characterized by stable aggregates, with numerous macropores in the surface and a predominantly microporous subsoil matrix i nterspersed with a few larger macropores. Macropores account for about 29% of porosity in the surface and between 3 and 6% in the subsoil. The saturat ed hydraulic conductivity of the matrix containing macropores averages abou t 300 to 400 cm/day, whereas that of the microporous matrix is generally <1 cm/day. The structure facilitates rapid infiltration and leaching of rainf all. However, little opportunity exists for nutrients moving downward with drainage water to accumulate in the subsoil. The main reason for this seems to be the low hydraulic conductivity and the, preponderance of excessive w etness in the subsoil, Drying seems to be essential for movement of nutrien ts into the subsoil matrix, However, most of the agronomic crops are sensit ive to Al toxicity and fail to grow roots deeper than: 10 to 15 crh. Thus, they suffer from water stress, despite heavy and frequent rainfall, and fai l to cause drying of the subsoil, Problems of acidity; AZ toxicity, and inf ertility worsen progressively where agricultural production consists mainly of Al-sensitive crops. Although liming with calcium carbonate improves the soil chemical environment, downward movement of lime is very slow. Deep: l iming is effective in improving rooting depth, crop water availability, and drying of the subsoil, but the technology is cost- and labor-intensive. Na tive vegetation, on the other hand, is able: to:grow roots to considerable depths and causes significant drying of the subsoil, even without soil amen dments. Thus, production systems: in which locally adapted vegetation of ec onomic value is the-main focus: seem to be more sustainable and conducive t o improving soil conditions.